Earliest Sunset

You may not have noticed it, but the Sun is setting a little later right now than it did a few days or weeks ago. Yet the days are continuing to get shorter, and will until after the winter solstice on Friday.

The difference is caused by the fact that there are two ways to figure the length of a day.

The one we use defines a day as exactly 24 hours. But the other way follows the motion of the Sun from one solar noon to the next — the moment the Sun is highest in the sky. Under this system, the length of a day can vary by about a half-minute from one day to the next.

There are a couple of reasons for that variation. One is Earth’s tilt on its axis, which causes the Sun to rise at different angles and at different locations along the horizon on different days. The other is Earth’s lopsided orbit, which causes the Sun to return to the same point in the sky a little earlier or later in the day depending on whether we’re close to the Sun or far away.

When you add these factors together, you find that the earliest sunsets for places like Honolulu and Miami came around the end of November. For higher latitudes, like Philadelphia, they came about a week ago. And for cities that are even farther north, like Anchorage, earliest sunset is happening about now.

And to balance things out, the date of latest sunrise comes after the solstice, working from north to south — a few more minutes of darkness before the dawn of another day.

Script by Damond Benningfield, Copyright 2012

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

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